Japanese Patent Publication (Kokoku) No. 1-30436 discloses an example of such a conventional ultrasonic sensor which senses obstacles around a vehicle using ultrasonic waves.
Japanese Patent Laid-Open (Kokai) No. 62-131813 discloses another example which senses the condition of a road ahead of a vehicle for the purpose of alleviating shocks caused to the body of the vehicle by road surface irregularities.
Conventionally, for the purpose of sensing an object such as an obstacle around a travelling vehicle by use of ultrasonic waves, it is common practice to calculate the distance between the vehicle and the object using ultrasonic waves, and to this end, various proposals have been made for accurately measuring the distance from a vehicle to an object which lies within a few centimeters or few meters away from the vehicle. In this practice, however, an ultrasonic sensor senses, in addition to reflecting waves from the object, those from a predetermined forward surface of a road, and to avoid this, it is a general trend to direct a horn to an appropriate orientation so as not to pick up undesirable waves reflected from the road surface. Thus, in this case, no feasible approach on signal processing has generally been made for positively sensing only the ultrasonic waves reflected from the road surface so as to detect obstacles thereon in an effective manner.
On the other hand, another proposal has been made in which continuous ultrasonic waves are projected toward the surface of a road ahead of a vehicle, and reflected waves therefrom are continuously monitored so as to detect obstacles on the road surface. In this case, however, there are many drawbacks as listed below.
1) Overheating of a wave emitter precludes the energy of ultrasonic waves emitted therefrom from being enhanced to a practical level.
2) Interference between the projected waves and the reflected waves takes place, and development of standing waves provides adverse effects.
3) It is impossible to discern between reflected waves from the road surface and reflected waves from other places or objects.
4) There is no effective measure for removing the influences of factors other than road surface irregularities such as, for example, winds, temperature variations, etc., which can change the intensity of reflected waves.
Accordingly, the above-mentioned conventional ultrasonic obstacle sensors have various problems as referred to above, and are not satisfactory and feasible for practical applications.